1. Field of the Invention
The present invention relates to methods of reproducing multilevel information from information recording media, such as optical disks. More specifically, the present invention relates to a method of processing multilevel data in which the error rate of data can be reduced.
2. Description of the Related Art
Generally, binary digital data is recorded on optical disks, on spiral or concentric tracks in the form of pits defined by projected and recessed portions, formed by embossing or the like (in the case of ROM disks). In other forms of recording, binary digital data is recorded in the form of holes formed on inorganic or organic recording films (in the case of writable disks), or in the form of differences in crystallization states (in the case of phase change disks). When such recorded data is reproduced, tracks are irradiated with laser beams, and differences in the intensity of reflected light or differences in the direction of polarization due to the magnetic Kerr effect are detected to obtain reproduced RF signals. Then, binary data is detected from the reproduced RF signals.
Recently, research and development is focused on increasing the density of recording on optical disks. With techniques for reducing the size of a light spot used for recording and reproducing information, the wavelength of light from a light source is changing from that of red light (650 nm) to that of blue violet (405 nm). Furthermore, the numerical aperture (NA) of an object lens is being increased from 0.6 or 0.65 to 0.85. At the same time, techniques for recording and reproducing multilevel information efficiently without reducing the size of a light spot have also been proposed.
For example, the assignee of this application proposed a method of recording and reproducing multilevel information in Japanese Patent Laid-Open No. 5-128530.
In a recording and reproducing method, multilevel information is recorded on information tracks of an optical information recording medium on the basis of combinations of the widths of information pits in the direction of the tracks and the amounts of shift of the information pits in the direction of the tracks relative to a light spot for reproduction. Furthermore, when the multilevel information recorded in the form of the information pits is reproduced, multilevel information is reproduced on the basis of a correlation between detection signals learned in advance and detection signals obtained from the light spot.
According to a report presented at ISOM 2003 (Writeonce Disks for Multi-level Optical Recording, Proceedings Fr-Po-04), which is an international symposium for researches in the field of optical disks, an optical system with a blue-violet light source (405 nm) and a numerical aperture (NA) of 0.65 is used.
The optical system records and reproduces 8-level multilevel information on and from an optical disk having a track pitch of 0.46 μm. On the optical disk, the width in the direction of the tracks of each virtually defined region for recording one information pit is 0.26 μm. The virtually defined region will hereinafter be referred to as a cell.
When multilevel data is recorded, information that has been converted from binary to 8 levels is recorded in each cell. More specifically, in the case of 8-ary recording, one cell corresponds to 3-bit binary data.
For example, 3-bit binary data and 8-ary levels can have the following relationships:
(0, 0, 0) corresponds to level 0.
(0, 0, 1) corresponds to level 1.
(0, 1, 0) corresponds to level 2.
(0, 1, 1) corresponds to level 3.
(1, 1, 0) corresponds to level 4.
(1, 1, 1) corresponds to level 5.
(1, 0, 0) corresponds to level 6.
(1, 0, 1) corresponds to level 7.
The widths of the information pits corresponding to the eight levels are defined as follows by equally dividing the width of each cell in the direction of the tracks as shown in FIG. 1.
Level 0 is represented by the absence of an information pit.
Level 1 is represented by a width of 2/16 of the cell width.
Level 2 is represented by a width of 4/16 of the cell width.
Level 3 is represented by a width of 6/16 of the cell width.
Level 4 is represented by a width of 8/16 of the cell width.
Level 5 is represented by a width of 10/16 of the cell width.
Level 6 is represented by a width of 12/16 of the cell width.
Level 7 is represented by a width of 14/16 of the cell width.
When information pits defined as described above are recorded randomly and the amounts of light reflected from the information pits are received by a photodetector, the amplitudes of signals reproduced from the information pits are distributed as shown in FIG. 2. The signals are sampled at timings when the center of the light spot comes at the centers of the widths of individual cells in the direction of the tracks.
Furthermore, an output of reproduced signals in the case where level 0 represented by the absence of an information pit is defined as “1”, and an output of reproduced signals in the case where information pits of level 7 are successively recorded is defined as “0”.
The value of a reproduced signal corresponding to each level has a certain width due to the effect of information pits preceding and succeeding subject information pits (i.e., intersymbol interference).
When the distribution of the amplitudes of reproduced signals overlaps between adjacent levels, it is not possible to achieve separation and detection with a fixed threshold.
According to the report presented in ISOM 2003, learning is executed to read and to store signals reproduced from pit sequences in which the value of a subject information pit and the values of preceding and succeeding information pits are known.
Then, signals reproduced from actual information pits are compared with the recorded values to achieve separation and detection. This serves to overcome the problem of intersymbol interference described above.
The assignee of this application proposed the following method of recording and reproducing multilevel information in Japanese Patent Application No. 2005-047198 as a technique for recording and reproducing multilevel information while suppressing intersymbol interference.
FIG. 3 shows a positional relationship between a light spot and preceding and succeeding cells in a case where a cell-center value is sampled. For example, the track pitch is 0.32 μm, the size of the light spot is 0.405 μm (the wavelength is 405 nm and the NA of an object lens is 0.85), and the size of a cell is 0.2 μM. It is experimentally known that, with these parameters, the cell-center value of the subject cell does not take on the same value when the levels of the preceding and succeeding cells are varied among 0 to 7 and have a certain width due to the effect of intersymbol interference.
The intersymbol interference can be understood intuitively from the fact that the skirts of the light spot on the middle cell partially overlap the left and right cells in FIG. 3. The effect of intersymbol interference increases as the size of the cell becomes smaller relative to the size of the light spot.
FIG. 4 shows a positional relationship at a timing when the light spot has come to a boundary between left and right cells in a case where a cell-boundary value is sampled. Since the size of the light spot is 0.405 μm and the width of two cells is 0.4 μm, most of the light spot is on the left and right cells. That is, the cell-boundary value sampled at the boundary between the left and right cells is not substantially affected from outside, so that the effect of intersymbol interference from outside the left and right cells is small.
FIGS. 5 and 6 are histograms showing the results of simulations of the levels of reproduced signals of cell-center values and cell-boundary values, respectively. The conditions of the simulations are as follows. An optical system has a blue-violet light source (405 nm) and an NA of 0.85, and an optical disk has a track pitch of 0.32 μm. The size of each virtually defined cell for recording one information pit is 0.20 μm, and multilevel data takes on values among 0 to 7.
As shown in FIG. 5, in the case of cell-center values, because of intersymbol interference, the levels of reproduced signals are not separated. In contrast, as shown in FIG. 6, the levels of reproduced signals of cell-boundary values are separated to fifteen values. Japanese Patent Application No. 2005-047198 describes a method of determining multilevel information on the basis of both the levels of reproduced signals of cell-center values and the levels of reproduced signals of cell-boundary values.
In this specification, the levels of reproduced signals of cell-boundary values separated to fifteen values will be referred to as levels 0 to 14 of cell-boundary values. For example, the lowest level of a reproduced signal in FIG. 6 will be referred to as level 0 of a cell-boundary value.
However, when the cell-boundary values are used for detection of multilevel information as described above, the following problems arise.
In optical disks, level variation or amplitude variation could occur due to various factors, such as difference in reflectivity among various types of optical disks or a difference in reproduction frequency characteristics between an inner side and an outer side of a single optical disk, as well as intersymbol interference. Thus, even when the method of separation and detection described above is used, reproduced signals could be detected incorrectly.
Particularly, since the number of levels of cell-boundary values is greater than the number of levels of cell-center values, the signal-to-noise ratio (S/N ratio) of cell-boundary values is more susceptible to the effects of factors other than intersymbol interference compared with that of cell-center values.